This application is related to Merck U.S. Pat. No. 4,663,321. Cholecystokinin (CCK) is a neuropeptide composed of thirty-three aminoacids in its originally isolated form. See: Mutt and Jorpes, Biochem. J. 125 678 (1971). Also occurring in circulation are 39, 12 and 8 amino acid forms. The carboxyl terminal octapeptide (CCK-8) is the minimum active sequence. Gastrin occurs in 34, 17, and 14 amino acid forms in circulation and is related to CCK by identity of the C-terminal pentapeptides Gly-Trp-Met-Asp-Phe-NH.sub.2. Gastrin and CCK exist in both gastrointestinal tissue and the central nervous system. V. Mutt, Gastrointestinal Hormones, G. B. J. Glass, Ed., Raven Press, N.Y., p. 169 and G. Nisson, ibid, p. 127.
The isolation of the 33-amino acid polypeptide, cholecystokinin (CCK-33), from porcine intestine, Mutt, V. et al., "Structure of Porcine Cholecystokininpancreozymin. 1. Cleavage with Thrombin and Trypsin", European J. Biochem. 6, 156, (1968), was followed by the discovery that it occurs in numerous molecular forms at various sites throughout the peripheral and central nervous systems, Larsson, L. et al., "Localization and Molecular Heterogeneity of Cholecystokinin in the Central and Peripheral Nervous System", Brain Res., 165, 201 (1979). In the mammalian brain the predominant fragments are the carboxy terminal octapeptide, H-Asp-Tyr(SO.sub.3 H)-Met-Gly-Trp-Met-Asp-Phe-NH.sub.2 (CCK-8s, CCK.sub.26-33) and tetrapeptide, CCK-4 (CCK.sub.30-33).
The carboxy terminal octapeptide possesses the full biological profile of CCK, Dockray, G. J. et al., "Isolation, Structure and Biological Activity of Two Cholecystokinin Octapeptides from Sheep Brain", Nature 274, 711 (1978), and meets many anatomical and biochemical criteria which characterize a neurotransmitter, Vanderhaeghen, J. J. et al., "J. Neuronal Cholecystokinin", Ann. N.Y. Acad. Sci., 448, (1985). The presence of high concentrations of CCK-8s in the mammalian CNS is complemented with findings of specific and high affinity membrane-bound CCK binding sites, Innis, R. B. et al., "Distinct Cholecystokinin Receptors in Brain and Pancreas", Proc. Natl. Acad. Sci. U.S.A., 77, 6917 (1980).
Evidence that more than one form of CCK receptor might exist was first provided in 1980 by Innis and Snyder, Innis, R. B. et al., "Distinct Cholecystokinin Receptors in Brain and Pancreas", Proc. Natl. Acad. Sci. U.S.A., 77, 6917 (1980). At present, CCK receptors have been differentiated into primarily two subtypes based on their affinity for CCK fragments and analogues, Innis, R. B. et al., "Distinct Cholecystokinin Receptors in Brain and Pancreas", Proc. Natl. Acad. Sci. U.S.A., 77, 6917 (1980). The subsequent development of agents which discriminate between different CCK receptor types afforded further support for these assignments, Chang, R. S. L. et al., "Biochemical and Pharmacological Characterization of an Extremely Potent and Selective Nonpeptide Cholecystokinin Antagonist", Proc. Natl. Acad. Sci. U.S.A., 83, 4923 (1986).
The CCK-A receptors, previously known as peripheral CCK receptors, are located in organs such as the pancreas, gallbladder, and colon. They exhibit high affinity for CCK-8s and a lower affinity for the corresponding desulphated fragment, CCK-8d, for CCK-4, and gastrin. Recent autoradiographic results have localized CCK-A receptors in the brain as well, Hill, D. R. et al., "Autoradiographic Localization and Biochemical Characterization of Peripheral Type CCK Receptors in Rat CNS Using Highly Selective Nonpeptide CCK Antagonists", J. Neurosci., 7, 2967 (1987).
The majority of the CCK receptors in the brain are of the CCK-B type. These were previously designated as central CCK receptors. CCK-B receptors are widely distributed throughout the brain and display high affinity for CCK-8s, CCK-4, and pentagastrin, Hill, D. R. et al., "Autoradiographic Localization and Biochemical Characterization of Peripheral Type CCK Receptors in Rat CNS Using Highly Selective Nonpeptide CCK Antagonists", J. Neurosci, 7, 2967 (1987).
In addition to the above mentioned CCK receptor subtypes is a third type, the stomach gastrin receptor, which appears to be closely related to the CCK-B receptor subtype, Beinfeld, M. C., "Cholecystokinin in the Central Nervous System; a Minireview", Neuropeptides, 3, 4111 (1983). The minimum fully potent CCK sequence at this receptor is CCK-4, Gregory, R. A., "A Review of some Recent Development in the Chemistry of the Gastrins", Biorg. Chem., 8,497 (1979).
A wide range of physiological responses has been attributed to CCK. In an effort to elucidate its biological roles, researchers have relied primarily on a collection of CCK-A antagonists which has been steadily supplemented and improved to now include very selective, high-affinity agents, Evans, B. E., "Recent Developments in Cholecystokinin Antagonist Research," Drugs Future, 14, 971 (1989). In addition to their value as investigative tools, CCK antagonists retain considerable therapeutic potential, Gertz, B. J., "Potential Clinical Applications, of a CCK Antagonist in Cholecystokinin Antagonists," Alan R. Liss, Inc.: New York, pp. 327 (1988).
In recent years, interest in agonists and antagonists of CCK has been stimulated by the possible clinical application of such compounds, Silverman, M. A. et al., "Cholecystokinin Receptor Antagonists, a Review", Am. J. Gastroenterol, 82, 703, (1987). The discovery of the presence of CCK in the brain and its significance in relation to its modulation of dopaminergic functions, effects on satiety, its roles in nociception, in anxiety, and other brain functions, Vanderhaeghen, J. J., et al., "J. Neuronal Cholecystokinin", Ann. N.Y. Acad. Sci. 448 (1985) has understandably intensified the search for CCK-B selective agents. Since the relevant biologically active fragment, CCK-8s, has a half-life of less than 1 hour, Deschodt-Lanckman, K., et al., "Degradation of Cholecystokinin-like Peptides by a Crude Rat Brain Synaptosomal Fraction: a Study by High Pressure Liquid Chromatography", Reg. Pept., 2, 15 (1981), implicit in the development of candidates for clinical use are criteria of high potency, selectivity, long in-vivo duration, oral bioavailability, and capability of penetrating the blood-brain barrier. These are strict prerequisites, given the tenuous stature of peptides as drugs, Veber, D. F., et al., "The Design of Metabolically-stable Peptide Analogs", Trends Neurosci. 8, 392 (1985).
Nevertheless, by employing stratagems which stabilize peptide structures, advances have been made toward developing highly potent and selective peptidal CCK-B receptor ligands Charpentier, B. et al., "Cyclic Chloleycstokinin Analogues with High Selectivity for Central Receptors". Proc. Natl. Acad. Sci. U.S.A.,85, 1968, (1988). Analogues are now available which have proven resistant to enzymatic degradation Charpentier, B. et al., "Enzyme-resistant CCK Analogs with High Affinities for Central Receptors", Peptides, 9 835 (1988). Despite favorable receptor binding profiles, this class of compounds fails to meet previously cited key requirements which characterize a drug candidate. In response, researchers have turned to non-peptide compounds which offer a broader range of structure and physicochemical properties.